A model system has been developed to study cell surface receptor proteins for type C retroviruses. Kinetics of the binding of viral envelope proteins to murine thymocytes have been determined and those cell surface molecules involved in the binding have been investigated by chemical cross-linking with a cleavable bifunctional reagent. Cross-linked complexes of R-MuLV proteins and thymocyte cell surface polypeptides have been isolated utilizing specific immunoprecipitation techniques and a determination of the molecular weights of polypeptides either comprising of or "nearest neighbors" to the binding site (receptor) for R-MuLV proteins have been made. The characterization and relationship of these molecules to products of the major histocompatibility locus is examined.